Re: [PATCH v19 QEMU 3/4] virtio-balloon: Provide an interface for free page reporting

[David Hildenbrand <david@redhat.com>]

On 10.04.20 05:41, Alexander Duyck wrote:
> From: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> 
> Add support for free page reporting. The idea is to function very similar
> to how the balloon works in that we basically end up madvising the page as
> not being used. However we don't really need to bother with any deflate
> type logic since the page will be faulted back into the guest when it is
> read or written to.
> 
> This provides a new way of letting the guest proactively report free
> pages to the hypervisor, so the hypervisor can reuse them. In contrast to
> inflate/deflate that is triggered via the hypervisor explicitly.

Much better, thanks!

> 
> Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> ---
>  hw/virtio/virtio-balloon.c         |   63 +++++++++++++++++++++++++++++++++++-
>  include/hw/virtio/virtio-balloon.h |    2 +
>  2 files changed, 62 insertions(+), 3 deletions(-)
> 
> diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
> index 1c6d36a29a04..86d8b48a8e3a 100644
> --- a/hw/virtio/virtio-balloon.c
> +++ b/hw/virtio/virtio-balloon.c
> @@ -321,6 +321,57 @@ static void balloon_stats_set_poll_interval(Object *obj, Visitor *v,
>      balloon_stats_change_timer(s, 0);
>  }
>  
> +static void virtio_balloon_handle_report(VirtIODevice *vdev, VirtQueue *vq)
> +{
> +    VirtIOBalloon *dev = VIRTIO_BALLOON(vdev);
> +    VirtQueueElement *elem;
> +
> +    while ((elem = virtqueue_pop(vq, sizeof(VirtQueueElement)))) {
> +        unsigned int i;
> +
> +        for (i = 0; i < elem->in_num; i++) {
> +            void *addr = elem->in_sg[i].iov_base;
> +            size_t size = elem->in_sg[i].iov_len;
> +            ram_addr_t ram_offset;
> +            size_t rb_page_size;
> +            RAMBlock *rb;
> +
> +            if (qemu_balloon_is_inhibited() || dev->poison_val) {
> +                continue;

actually, you want to do that in the outer loop, no?

> +            }
> +
> +            /*
> +             * There is no need to check the memory section to see if
> +             * it is ram/readonly/romd like there is for handle_output
> +             * below. If the region is not meant to be written to then
> +             * address_space_map will have allocated a bounce buffer
> +             * and it will be freed in address_space_unmap and trigger
> +             * and unassigned_mem_write before failing to copy over the
> +             * buffer. If more than one bad descriptor is provided it
> +             * will return NULL after the first bounce buffer and fail
> +             * to map any resources.
> +             */
> +            rb = qemu_ram_block_from_host(addr, false, &ram_offset);
> +            if (!rb) {
> +                trace_virtio_balloon_bad_addr(elem->in_addr[i]);
> +                continue;
> +            }
> +
> +            /* For now we will simply ignore unaligned memory regions */
> +            rb_page_size = qemu_ram_pagesize(rb);
> +            if (!QEMU_IS_ALIGNED(ram_offset | size, rb_page_size)) {

/me thinks you can drop rb_page_size

I *think* there is still one remaining case to handle: Crossing RAM blocks.

Most probably you should check

/* For now, ignore crossing RAM blocks. */
if (ram_offset + size >= qemu_ram_get_used_length()) {
	continue;
}

otherwise ram_block_discard_range() will report an error.

-- 
Thanks,

David / dhildenb